I have an older semi truck that I'm having A/C issues with. The truck is running 134a and every component of the system has been replaced except the condenser (expansion valve, compressor is new Sanden, evaporator, etc). The A/C works good at idle, but after driving for a while it will still work but warm up by a good 10 degrees. If you turn it off for a little while it will start working again but soon do the same thing. This is opposite of every air conditioning system I've been around. Pressures are running around 30 on the low side and 225 on high side at roughly 90 degrees. I've pulled it back down and recharged to make sure nothing is wrong there but no difference. Also changed dryer and expansion valve a couple of times trying to figure it out. Can someone tell me what's wrong and what to try? Thanks,

You might be a little over charged. Also does this truck have a bunk A/C/heater? If so try running with both the cab and bunk system on and see if the temp is better. I guess the next question is what has been changed from stock? If it was R12 to start with, than the stock amount will be too much for 134a, If it was 134a to begin with that shouldn't be a problem. When the engine speeds up and there is a slight overcharge, the excess gets stored as high pressure liquid on the high side and can back up into the condenser reduceings its capacity to shed heat from the gas. Your presures don't look out of line for the temps you reported. If all else fails, you could try increseing the condenser capacity. This would mean useing a larger generic unit in the 2-3 ton range. I don't suggest going that route unless all else fails, as your pressures don't indicate a lack of heat sheding My first thing would be to be sure of the amount of 134a by weight. Another thing to check is that the heater control is fully shutting off heat from the cabin air. Most trucks have shut off valves at the engine that you can shut off all flow as a test and see if it cools better, if so look to why the dash control is allowing some heat into the cabin.

It does have a bunk A/C unit that has been added on at some point in the past. It ties into the existing A/C system and will warm up/cool down along with the cab A/C. No one has been able to give me any specs on what amount this system held from the factory, and that combined with the sleeper unit being added on keeps me from being able to weigh the refrigerant putting it in so I have to depend on the gauges. It was R12 originally ('78 model truck) and had a York compressor. There are valves on the heater hoses that I keep shut off. It really doesn't cool much at all with them open and letting water circulate. The condenser looks huge to me, it takes up the entire grill of the truck and sits out away from the radiator (big air gap between the two), but who knows. I forgot to mention I did flush the system too but it wasn't dirty.Would you suggest just pulling some refrigerant out and seeing what it does?

I've wrestled with what you are. (no known amount, modified system, how much to you use?) and asked here and the answer is you have to experiment to find what works best, then hopefully be able to write down that amount for future use. My experience is those honking big condensers, that were designed for R-12 with wide fins and large tubes (to minimize air flow restriction to the radiator) don't handle 134a as well as the modern parallel flow multi path condensers that are physically smaller but have a higher capacity (like the MEI 6314, it has almost 32000 BTU Cap). I replaced the one on my IHC 9670 that filled the whole grill opening yet didn't cool well on 134a with the 6314 and I have COLD A/C. When you are un sure of how much to 134a to use, it is always better to start out low, and work up then to put too much in. If you have a sight glass in the receiver, you can use that as a guide. Always allow some time for the system to "settle" between adding. Be prepared to go back and tweek it esp if on a really hot day. As long as you have liquid in the sight glass, you should be close, if there is foam or lots of bubbles you are low, too high will look the same as "just right" so work up to it from too low, not down from too high. With my engine fan running, the temps in the mid to upper 90's in a hot parking lot, the inlet to the condenser is HOT but the outlet is just warm, slightly above ambient That shows that amost all the heat is removed from the 134a. Be glad you have an old system with a reciever and Tx valve. They are a lot more forgiving of amount than are orifice tube systems. No condenser will work well without good air flow, but if you are cooling ok at idle, than I don't think airflow is a problem. Added info: Make your adjustments with both bunk and cab units on. Sight glass can be misleading if the condenser is not doing its job. Compare pressure vs. temp and see if it is in the ball-park. There is a pressure/temp chart for both 134a and R-12 on this fourm. If the air across the condenser is 90 deg, you want to see a high side pressure of no more than 40 deg higher (130 deg ) or around 225 psi for 134a. The lower you can get the high side temp/pressure the better cooling. In otherwords, the more heat the condenser can remove, the lower the high side condensing temp/pressure and the better it will cool. So if you can get the outlet of the condenser to be 105-110 deg F (instead of 130F), the pressure will be 135-147.The same holds for low side. compare you vent temp with the low side pressure

Thanks for the help. I've been thinking I'm just gonna have to experiment with this too and see what works since I'm out of stuff to replace. One thing I've wondered, do any of you all know if there's a chance of finding a higher output blower? The one in the cab doesn't put out much air but has been replaced also. If I can get the air to hold a steady temp and blow harder I'll be doing great.

ParkerFly wrote:Thanks for the help. I've been thinking I'm just gonna have to experiment with this too and see what works since I'm out of stuff to replace. One thing I've wondered, do any of you all know if there's a chance of finding a higher output blower? The one in the cab doesn't put out much air but has been replaced also. If I can get the air to hold a steady temp and blow harder I'll be doing great.

Make sure that your evaporator (inside its case) is not blocked by leaves, debris, dirt, blocking air flow. Make sure that some air is not deflected underdash, up near windshield.

As to warmer temperatures while driving after a while, make sure that the AC clutch is actually still engaged when this happens, to eliminate a gap or slippage issue.

I don't know of any high capacity replacements, but on one of my trucks, that doesn't have a filter before the cores, the heater core was covered with a combo of dog fur and dirt. Cleaning both the evaporator and heater core made a big difference in air flow The IHC cabover truck have a filter before the cores, and it amazing how quick it can load up. What brand and model truck are you working on?

This is a '78 Kenworth W900. The cores both sit above the passengers feet, and I pulled the cover off a few weeks ago to look and both looked extremely clean and relatively new. I'm sure they've been replaced in the last few years. I may check again though to make sure they are clean and not just look clean.